Internet Engineering Task Force (IETF) J. Levine
Request for Comments: 7505 Taughannock Networks
Category: Standards Track M. Delany
ISSN: 2070-1721 Apple Inc.
June 2015
A "Null MX" No Service Resource Record for Domains That Accept No Mail
Abstract
Internet mail determines the address of a receiving server through
the DNS, first by looking for an MX record and then by looking for an
A/AAAA record as a fallback. Unfortunately, this means that the
A/AAAA record is taken to be mail server address even when that
address does not accept mail. The No Service MX RR, informally
called "null MX", formalizes the existing mechanism by which a domain
announces that it accepts no mail, without having to provide a mail
server; this permits significant operational efficiencies.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7505.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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described in the Simplified BSD License.
Levine & Delany Standards Track [Page 1]

RFC 7505 Null MX June 2015
The terms "RFC5321.MailFrom" and "RFC5322.From" are used as defined
in [RFC5598].
3. MX Resource Records Specifying Null MX
To indicate that a domain does not accept email, it advertises a
single MX RR (see Section 3.3.9 of [RFC1035]) with an RDATA section
consisting of preference number 0 and a zero-length label, written in
master files as ".", as the exchange domain, to denote that there
exists no mail exchanger for a domain. Since "." is not a valid host
name, a null MX record cannot be confused with an ordinary MX record.
The use of "." as a pseudo-hostname meaning no service available is
modeled on the SRV RR [RFC2782] where it has a similar meaning.
A domain that advertises a null MX MUST NOT advertise any other MX
RR.
4. Effects of Null MX
The null MX record has a variety of efficiency and usability
benefits.
4.1. SMTP Server Benefits
Mail often has an incorrect address due to user error, where the
address was mistranscribed or misunderstood, for example, to
alice@www.example.com, alice@example.org, or alice@examp1e.com rather
than alice@example.com. Null MX allows a mail system to report the
delivery failure when the user sends the message, rather than hours
or days later.
Senders of abusive mail often use forged undeliverable return
addresses. Null MX allows Delivery Status Notifications (DSNs) and
other attempted responses to such mail to be disposed of efficiently.
The ability to detect domains that do not accept email offers
resource savings to an SMTP client. It will discover on the first
sending attempt that an address is not deliverable, avoiding queuing
and retries.
When a submission or SMTP relay server rejects an envelope recipient
due to a domain's null MX record, it SHOULD use a 556 reply code
[RFC7504] (Requested action not taken: domain does not accept mail)
and a 5.1.10 enhanced status code (Permanent failure: Recipient
address has null MX).
Levine & Delany Standards Track [Page 3]

RFC 7505 Null MX June 2015
A receiving SMTP server that chooses to reject email during the SMTP
conversation that presents an undeliverable RFC5321.MailFrom or
RFC5322.From domain can be more confident that for other messages a
subsequent attempt to send a DSN or other response will reach a
recipient SMTP server.
SMTP servers that reject mail because a RFC5321.MailFrom or
RFC5322.From domain has a null MX record SHOULD use a 550 reply code
(Requested action not taken: mailbox unavailable) and a 5.7.27
enhanced status code (Permanent failure: Sender address has null MX).
4.2. Sending Mail from Domains That Publish Null MX
Null MX is primarily intended for domains that do not send or receive
any mail, but have mail sent to them anyway due to mistakes or
malice. Many receiving systems reject mail that has an invalid
return address. Return addresses are needed to allow the sender to
handle message delivery errors. An invalid return address often
signals that the message is spam. Hence, mail systems SHOULD NOT
publish a null MX record for domains that they use in
RFC5321.MailFrom or RFC5322.From addresses. If a system nonetheless
does so, it risks having its mail rejected.
Operators of domains that do not send mail can publish Sender Policy
Framework (SPF) "-all" policies [RFC7208] to make an explicit
declaration that the domains send no mail.
Null MX is not intended to be a replacement for the null reverse-path
described in Section 4.5.5 of RFC 5321 and does not change the
meaning or use of a null reverse-path.
5. Security Considerations
Within the DNS, a null MX RR is an ordinary MX record and presents no
new security issues. If desired, it can be secured in the same
manner as any other DNS record using DNSSEC.
Levine & Delany Standards Track [Page 4]